My name is Kim Martinez and I am 25 years old.. (Using double quotes)
My name is Kim Martinez and $age (Using single quotes)
One of the main differences between using single quotation marks vs double quotation marks is that when using single quotation marks everything inside the quotation marks are displayed as is. For example, the variable displayed above is not interpreted to its value. When the same statement is displayed in doube quotation marks the variable is displayed as its value.